Windows/Mac only: Freeware system tray application Calq adds an on-demand calculator to your computer. Once installed, you can pull up Calq at any time through a quick keyboard shortcut, run your calculation, then hide it just as easily. The simple, one-dimensional Calq is freeware, Windows and Mac OS X only. Since this little app obviously retreads territory that's already available in many other incarnations, hit the jump for a rundown of other quick methods of calculations you might want to add to your repertoire instead of installing Calq.

If you're a Windows users, you can pull up a quick calculator in a ton of ways, but here are a few:

If you're using the popular application launcher, Launchy, just invoke Launchy and type your simple calculation into Launchy. It automatically evaluates the calculation on-the-fly.

If you're on a Mac, you've also got several quick alternate options.

Spotlight does on-the-fly calculations just like Launchy, so just invoke Spotlight and get calculating.

Quicksilver also has a calculator action plug-in which allows you to run calculations through it's interface. If you've got the plug-in installed, invoke Quicksilver, hit period (.) to enter text mode, enter your calculation, then tab to the action pane and choose the CalculatorCalculateAction. It's not as quick or on-the-fly as Spotlight or Launchy, but it does give you a result to work with that you can easily copy to your clipboard.

Like Windows, OS X comes with a built-in calculator app which you can quickly launch using Spotlight or Quicksilver.

Of course, if for whatever reason you don't like these other options or you just like the idea behind Calq, it may be just the ticket for you. Got a favorite method of quickly running numbers on your computer that I didn't mention? Let's hear your preferred method of calculating in the comments.